1. Field of Invention
The present invention relates to an image forming apparatus provided with a fixing unit for fixing images formed on paper and a method of controlling heaters of the same.
2. Description of Related Art
Conventionally, a fixing unit has been proposed which includes a pressure heater for heating a pressure roller, a first and a second heater for heating a fixing roller together with the fixing belt running around the fixing roller. In accordance with this fixing unit, the pressure heater and the first and second heaters are controlled in order not to be concurrently on (energized) so that the maximum power consumption is kept down to suppress fluctuation in voltage and avoid the occurrence of a flicker, as described in Japanese Patent Published Application No. 2006-79055.
However, if there is a timing skew in the synchronized on/off operations of the heaters in the conventional image forming apparatus provided with a fixing unit, even though the skew is very small, voltage fluctuation may occur with asynchronous operations of the heaters to cause a flicker. For an instance, it is assumed that the first and second heaters are located adjacent each other. In this case, since these heaters are controlled in order not to be concurrently on, one of the heater remains off while the other heater is on, such that the area of the fixing roller to be heated by the one heater is inadvertently heated by the other heater which is on. On the other hand, these heaters are turned on/off by detecting the temperature of the areas to be heated by these heaters respectively. For example, a heater is turned on when the area to be heated by the heater cools down to a predetermined ON temperature, and also turned off when the area to be heated by the heater is heated to a predetermined OFF temperature. Because of this, since one heater which is on is inadvertently heating the area of the fixing roller to be heated by the other heater which is off, this area may not cool down to the ON temperature even after the one heater is turned off at the OFF temperature, such that the other heater is sometimes not turned on in this situation to disturb synchronization. This results in an asynchronous operation causing voltage fluctuation which in turn causes flickers. Meanwhile, this problem is not limited to the case where a plurality of heaters are adjacent each other but also applied to the case where a plurality of heaters are distant from each other because voltage fluctuation commonly occurs when these heaters are turned on/off asynchronously.
To achieve at least one of the abovementioned objects, an image forming apparatus reflecting one aspect of the present invention is provided with a fixing unit which fixes an image formed on a paper sheet by passing the paper sheet between a pair of rotatable members and comprises: a plurality of heaters configured to heat at least one of the rotatable members; a plurality of temperature detection units configured to detect the temperatures of areas to be heated by said plurality of heaters respectively; and a control unit configured to turn off each of the plurality of heaters when the temperature detected by one of the temperature detection units corresponding to the each heater rises to an OFF temperature, and turn on each of the plurality of heaters when the temperature detected by one of the temperature detection units corresponding to the each heater falls to an ON temperature, wherein if one of said plurality of heaters is on when another of said plurality of heaters cools down to the ON temperature, the control unit turns off the one heater and turns on the another heater at the same time when the area to be heated by the one heater is heated to the OFF temperature.
Also, to achieve at least one of the abovementioned objects, a heater control method for an image forming apparatus, which is provided with a fixing unit which fixes an image formed on a paper sheet by passing the paper sheet between a pair of rotatable members, comprises: a heating step of heating at least one of the rotatable members with a plurality of heaters; a temperature detecting step of detecting the temperatures of areas to be heated by said plurality of heaters respectively; and a control step of turning off each of the plurality of heaters when the temperature detected by one of the temperature detection units corresponding to the each heater rises to an OFF temperature, and turning on each of the plurality of heaters when the temperature detected by one of the temperature detection units corresponding to the each heater falls to an ON temperature, wherein if one of said plurality of heaters is on when another of said plurality of heaters cools down to the ON temperature, in the control step, the one heater and the another heater are turned off and on respectively at the same time when the area to be heated by the one heater is heated to the OFF temperature.